and ~25 kDa by SDS-PAGE. With a predicted protein core molecular weight of 12.3 kDa, it is possible that mobility is partially retarded by lacritin's amphipathic alpha helices. Predicted pI of lacritin's core protein is 5. Lacritin is subject to crosslinking by tissue transglutaminase, thereby giving rise to lacritin multimers including dimers and trimers. Crosslinking is initiated within 1 min in vitro, requiring as little as 0.1 nM lacritin. The ~0.6 micro molar level of tissue transglutaminase estimated in human tears is sufficient to promote crosslinking. Crosslinking involves the donors lysine 82 and 85 and the acceptor glutamine 106. Glutamine 106 resides within the amphipathic alpha helix near the C-terminus responsible for binding the N-terminus of syndecan-1. Accordingly, crosslinked lacritin binds syndecan-1 poorly and is inactive. Several lacritin splice variants have been detected in Aceview, from NEIBank EST data. Lacritin-b (11.1 kDa; pI 5.3) lacks the sequence SIVEKSILTE. Lacritin-c (10.7 kDa; pI 4.6) displays a novel C-terminus that should be incapable of binding syndecan-1, and lacks cell survival activity. Splice variants are proteoforms. Proteoforms include proteolytically processed forms of lacritin. Top down mass spec sequencing revealed that human tears contain five N- and forty-two different C-terminal lacritin-a proteoforms. Some approximate the bioactive lacritin synthetic peptides 'N-104', 'N-94' and 'N-94/C-6' from lacritin's C-terminus. Protease inhibitor studies suggest that processing of lacritin into C-terminal proteoforms requires a variety of tear proteases including cathepsin B, calpain, alanyl amino peptidase, arginyl aminopeptidase, MMP9, MMP10, cathepsin G, plasma kallikrein, plasmin, thrombin and trypsin. C-terminal proteoforms, like intact lacritin, are selectively deficient in dry eye tears. Cell targeting Lacritin targets a restricted group of epithelial cells (including human corneal epithelia), and not fibroblastic, glioma, or lymphoblastic cells. Cell surface proteoglycan syndecan-1 is partly responsible. Biotinylated cell surface proteins from a lacritin-responsive cell were incubated with lacritin under conditions of physiological salt. Those that bound lacritin were sequenced by mass spectrometry. Few bound. The most prominent was syndecan-1 (SDC1). In confirmatory pull-down assays, binding was not shared with family members syndecan-2 or syndecan-4, indicating that the protein core (and not the negatively charged heparan sulfate side-chains) was the main site of binding. Further analysis narrowed the site to syndecan-1's N-terminal 51 amino acids, and subsequently to the N-terminal sequence GAGAL that is conserved in syndecan-1's from different species. GAGAL promotes the alpha helicity of lacritin's C-terminal amphipathic alpha helix and likely binds to the hydrophobic face. Syndecan-1